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Bacteriological Culture and California Mastitis Test Results of Non-Clinical Quarters from Cows with Clinical Mastitis
The economic value of lactation therapy of subclinical mastitis with antibiotics is generally considered to be limited because of the cost of milk discarded during the withdrawal period. However, the treatment of quarters affected with subclinical mastitis when treating other quarters affected with clinical mastitis does not result in additional discarded milk. The objectives of this study were: a) describe bacteriological culture results of non-clinical quarters in cows affected with clinical mastitis; and, b) validate the California Mastitis Test (CMT) to identify subclinically infected quarters in these cows. A total of 109 cows with clinical mastitis from two dairy herds were identified with clinical mastitis at the general parlor and moved to a hospital pen where they were milked at a hospital parlor. At the first milking at the hospital parlor the CMT was performed and a milk sample was aseptically collected for culture from all quarters (clinical and non-clinical). Bacteria were isolated from 52% of the 110 quarters affected with clinical mastitis and from 35% of the other 319 non-clinical quarters. Coliforms (35%), non-agalactiae Streptococcus (25%), and coagulase-negative Staphylococcus (23%) were the bacteria most commonly isolated from clinical quarters. In non-clinical quarters coagulase-negative Staphylococcus (49%), Bacillus spp. (21%) and non-agalactiae Streptococcus (13%) were the bacteria most commonly isolated. Of all cows with clinical mastitis, 67% of them had bacteria isolated from at least one of the three non-clinical quarters. If also considering the quarter affected with clinical mastitis, 82% of the cows had at least one quarter infected. The CMT was evaluated for three diagnostic interpretations: identify bacterial growth, identify Gram-positive bacterial growth and identify non-agalactiae streptococci infections. The sensitivity of the CMT was 38%, 43% and 29%, and the specificity 71%, 70% and 68% for each one of the diagnostic interpretations, respectively. Therefore, the CMT was not a sensitive enough tool for the identification of subclinically infected quarters in cows with clinical mastitis. The high proportion of infected non-clinical quarters in cows with clinical mastitis may warrant the need to evaluate the efficacy and cost-benefit of treating clinical and subclinical quarters simultaneously.
Keywords: clinical mastitis, subclinical mastitis, California Mastitis Test